The present invention relates generally to material handling devices and more particularly to an improved boom structure for devices such as mobile excavators.
One type of known material handling device, which has been in existence for some time, includes a boom carried by a support for pivotal movement in a substantially vertical plane about a horizontal pivot axis. Generally, the support or platform is rotatable about a vertical axis on a self-powered frame structure and the boom is pivoted about the horizontal pivot axis on the platform by fluid powered means.
The free end of the boom pivotally supports a material handling unit, such as a dipper stick having a bucket pivoted thereon. The dipper stick is again pivoted relative to the free end of the boom through additional fluid power means while the bucket is pivoted on the outer end of the dipper stick through further fluid power means, such as fluid rams.
Self-powered units of this type have found a remarkable degree of commercial success. However, the construction and arrangement of the various parts in commercial machines heretofore known has placed limitations on the overall operation of the machine.
In the machines of the above-mentioned type, it is many times desirable to increase the overall length of the boom with respect to the platform to thereby increase the reach of the material handling unit with respect to the platform. Heretofore this has been accomplished in numerous different ways, such as the provision of telescopic boom sections that can be extended and retracted relative to each other to vary the effective length of the boom. While this arrangement has to some measure been satisfactory, there are certain shortcomings to such a solution. For example, in order to have telescoping boom sections, the overall weight of the boom must be substantially increased. Furthermore, utilizing telescoping boom sections for the mobile unit, results in decreasing the overall load capacity for the boom as the effective length of the boom is increased. The primary reason for such decrease in load capacity results from the overall weight of the boom sections, particularly when they are in an extended condition relative to the platform.
Another problem encountered with extending the overall length of the boom is the fact that the overall dimensions of the vehicle must be maintained within certain limmits in order for the vehicle to be transported on public highways.
While numerous alternates to the telescoping boom arrangement have been proposed, none of these have found any degree of commercial success, primarily because of the overall cost of manufacturing such units and also possibly because of the complicated arrangements that have been proposed for such units.
Thus, there remains a need for a structurally simple and compact mobile excavator which is capable of varying the effective length of the boom in a simple manner from the operator's compartment of the vehicle.